1. Field of the Invention
The present invention relates to a magnetic resonance tomography apparatus, in particular an iron-guided or permanently magnetic, short or open magnetic resonance (MR) tomography system, with thermally highly sensitive components and a temperature controller.
2. Description of the Prior Art
In iron-guided or permanent magnet magnetic resonance tomography systems, there are thermally highly sensitive components which strongly influence the homogeneity of the background field. Permanent magnets, shim plates or the like are examples of this. The temperature of these components must therefore be kept stable, since compensating the influence of these usually large-area components by means of software is possible only under certain conditions.
For example, a dynamic heat input arises due to fluctuations in room temperate or cooling water temperature, due to the time-dependent ohmic losses in the gradient coils, or due to eddy current losses in the components themselves. In the case of applications presently planned in an open MR system, the component temperature is permitted to fluctuate only by less than 0.5 K/10 min, it being necessary in this case to take account of the fact that power inputs in the range of 200-300 W/m2 are already a reality. Substantially more stringent requirements are expected in the future.
The problem of temperature-dependent inhomogeneity arises, firstly, when iron is used for field guidance, i.e., in the case of short or open MR systems, and secondly in the case of systems where permanent magnets are used for field generation. Moreover, some applications require extremely high field homogeneities such as, for example, in the case of spectroscopy. Equipment exhibiting these specifications is barely known at the moment, since the conventional stabilization of the temperature by water cooling or by means of heating cartridges does not permit the required sensitive correction, specifically in the case of permanent magnet systems.
It is an object of the present invention to configure a magnetic resonance tomography apparatus of the type initially described so as to ensure that the temperature can be kept constant in a fashion that can be sensitively controlled and stabilized with particular accuracy, thereby to ensure a particularly high level of homogeneity of the basic field.
This object is inventively achieved in a magnetic resonance tomography apparatus a foil heater which is provided with a control device and can be controlled in opposition to the internal dynamic heat sources.
According to the invention, the temperature is kept stable not by cooling mechanisms, but by an active auxiliary heater. The heater is switched on in the initial state, i.e., when no dynamic heat sources are present. During operation under load, that is to say when dynamic heat sources tend to influence the corresponding component temperatures, the corresponding heat input is compensated by decreasing to output of the active heater. The heater must react very quickly in order to ensure the effectiveness of this compensation, i.e., the heater must be situated close to the component and exhibit a spatial distribution corresponding to the discrepancies to be expected. All this is accomplished by the foil heater provided according to the invention which, in addition, does not unnecessarily enlarge the installation space, as is the case with a water cooler or in the use of heating cartridges.
In an embodiment the foil heater can be of bifilar design, and therefore no disturbance fields can be caused by the temperature compensation heater.
In order to be able to react particularly quickly to possible disturbing heat sources, and to compensate these by counteracting control, in a further embodiment of the invention the foil heater is arranged directly in the region of the heat sources formed by permanent magnets, shim plates or the like, for example between the shim plates and the neighboring gradient coils.
In order to be able to keep the control device as simple as possible, in another embodiment the invention the control device exhibits preliminary compensation on the basis of calculated and/or predetermined heat sources, so that the temporal and spatial characteristics of the heat sources, and thus the current characteristic required for the compensation, are determined for the foil heater in the control device by appropriate algorithms from the previously known current sequences of the gradient coils. Only a few feedback sensors are then still required, and the corrections caused by them are likewise only very slight.
In addition to the advantage of a quick thermal response to such a foil heater, and a good control response, the temperature controller according to the invention, by means of an active foil heater, has the advantages that there is a need for only a small amount of space, that no disturbance fields occur due to the bifilar arrangement, and that there is a good possibility of being able to adapt the local heat output in conjunction with a low price for the arrangement. It is particularly advantageous in this case to be able to adapt the local heat output to the monetary disturbing sources, and thisxe2x80x94apart from the very much larger spatial requirementxe2x80x94is not the case with a water cooler or with counteracting control by heating cartridges.